This invention relates to tamper-resistant fasteners and, more particularly, to tamper-resistant fasteners of the "specialty head" type.
Fasteners which resist removal by conventional means, such as screw drivers, hex keys, and the like are generally known in the art. Typically, such tamper-resistant fasteners are one of three general types: a one-way head, a breakaway head, or a specialty head. The one-way head fastener is typically driven in one direction by conventional means but cannot be driven in the opposite direction. Thus, such fasteners are essentially permanent once installed. Similarly, the breakaway head fastener is typically driven by conventional tools but can only be used once because a portion of the head breaks away from the fastener after use. The last of the three types, the specialty head fastener, is characterized by an unusually shaped recess in the head of the fastener into which is inserted a special tool having a suitably shaped end for the transmission of torque to the fastener. The specialty head fastener is the only one of the three types which is capable of multiple use.
Specialty head fasteners, to which the present invention relates, are generally known in the art. Examples of such fasteners can be found in U.S. Pat. Nos. 3,369,441; 4,258,596; and 2,338,023. Known specialty head tamper-resistant fasteners, however, are not without their disadvantages. One common problem with specialty head fasteners is that as the shape of the recess or socket in the fastener head becomes more intricate and complex, the more difficult, time-consuming and expensive it is to manufacture the fastener and the tool. For example, U.S. Pat. No. 4,258,596 discloses a tamper-resistant fastener having a socket comprised of three circular recesses, at least one of which is eccentrically disposed with respect to the others. This fastener requires multiple drilling operations and the tool requires multiple turning operations which increase the time and expense of manufacturing.
Another disadvantage of the known specialty head fasteners is that common manufacturing processes do not lend themselves to produce the close tolerance limitations required for proper operation of the fastener. Consequently, a poor fit results between the tool and the socket which causes sticking or jamming of the tool when torque is applied to the fastener.
I have invented a new specialty head type tamper-resistant fastener which is exceptionally difficult to operate except by use of a specially designed tool and is thus highly tamper-resistant and yet can be manufactured to close tolerances by common manufacturing processes and is therefore cheaper and easier to manufacture than known tamper-resistant fasteners.